Use gmtime_r instead of gmtime when compiling for posixy platforms

[horenmar]

Description

gmtime has the rather obvious problem of not being reentrant, and thus being rather unsafe to use. Catch2 does not support threads in its internals, but this is not enough to avoid the danger: if user uses gmtime inside the tested function, then our use of gmtime in the JUnit reporter will corrupt their state.

However, it took until C11 to get a standard alternative, which is gmtime_s, and which is only available under specific conditions (__STDC_WANT_LIB_EXT1__ has to be defined before the first inclusion of time.h). Instead we use gmtime_r, which while standardized only by C23, has been part of POSIX for ages.

While there are no nice compatibility guarantees for using gmtime_r, it should likely be compatible with all major supported non-Windows platforms.

GitHub Issues

Closes #2008